Sessional_Paper_1949 — Page 44

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Theories of spontaneous combustion are always to be formulated and treated with considerable reserve, as the process is a comparatively rare one. However, celluloid is one of the materials in which this phenomenon has been frequently observed, and studied to a considerable extent.

In the present case, due to a lucky combination of circumstances, it has been possible to develop the hypothesis of self-ignition and to account for the origin of the fire with reasonable certainty.

(a) Composition of nitrate motion picture film and washed film.

Nitrate motion picture film is made from a variety of celluloid; that is, a mixture of nitrocellulose and camphor, roughly in the proportions of 5 to 1. To this are added small quantities of plasticizing and stabilizing agents, some of these being in the nature of "trade secrets".

Upon this is coated the "emulsion", the sensitized gelatine surface which records the image; and often, in addition, other coatings are laid upon commercial film, in order to prolong its life, reduce its fire hazard, or for other reasons.

Every year the great motion picture companies accumulate very large quan- tities of old film, in the form of studio waste and of unserviceable film returned from their cinema circuits. This material is recovered by a process of washing, which removes the gelatin emulsion and leaves the original celluloid base. This material is known as "washed film”.

Washed film is not normally used again for making fresh film, as it is too variable to meet the exceedingly high quality standards of the industry, but it is sold for incorporation in cellulose finishes or to the celluloid plastic industry.

(b) Storage properties of celluloid; decomposition and stability. Nitrocellulose, the main constituent of nitrate film, is not a completely stable compound. It continually undergoes a chemical change akin to hydrolysis, whereby oxides of nitrogen are evolved. The reaction, under ordinary conditions, proceeds at an extremely slow rate, but may be accelerated by certain conditions.

For example, the reaction is accelerated by acids, and it is here that one danger lies. For the oxides of nitrogen evolved in the reaction change with traces of moisture to nitric acid, and this acid, as we have said, accelerates the reaction, again producing more acid. The reaction is, in chemical language, auto-catalytic.

The second danger of the reaction is that it is exothermic; that is, heat is evolved during the process. When the reaction is slow, this heat is of no significance; but as the reaction accelerates, particularly in a tight mass of celluloid or nitro- cellulose, the heat may build up to the stage where more is produced than can be dis- sipated by conduction or convection through the material. In this case the mass will, quite suddenly, burst into flame.

The third danger of this reaction is that it proceeds with great violence at temperatures insufficient to inflame ordinary combustible material. Thus at 170-180 degrees Centigrade, celluloid almost immediately inflames, and at much lower tempera- tures (130-140°C.) combustion will suddenly take place after a period measurable in hours rather than days.

It would appear from this that celluloid was too dangerous a commodity for mere humanity. But these are laboratory results; in practice a number of other considerations enter. For example, in a hermetically sealed vessel (such as a bomb) the oxides of nitrogen produced in the reaction are reduced to harmless nitrogen, and the reaction stops. In the presence of plenty of air, for example, in a box of celluloid toys, the oxides of nitrogen are dissipated and do no harm.

A constant small supply of air is necessary for the reaction of decom- position; and, in passing, this condition is admirably fulfilled by a well-closed but not hermitically sealed drum.